Heat oíl ín the same skíllet used for the oníons untíl the oíl begíns to smoke. Use a spatula to transfer the pattíes to the skíllet and cook 3 mínutes (wíthout movíng them). Flíp the pattíes and cook 1 more mínute. Place 1 cheese slíce over each patty and contínue to cook untíl cheese ís melted, about 20-30 seconds more.